Heat treatment



Nov, 6, T934.' W. sk. Bom/LANG HEAT TREATMENT Filed June 2. 1932 W. S. BOWLING HEAT TREATMENT Nov. 6, 1934.

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INVENTOR Patented Nov. 6, 1934 UNITED STATES HEAT TREATDIENT Wilbur S. Bowling, Salem, Ohio, assignor to The Electric Furnace Company, Salem, Ohio, a corporation of Ohio Application June 2, 1932, Serial No. 614,912

3 Claims.

This invention relates to processes wherein metals are subjected to temperatures at which oxidation takes place in contact with air, and more particularly to methods of carrying out 5 such processes wherein the metals or metal articles being heated are enveloped in a protective gas for the purpose of preventing such oxidation. The invention relates more particularly to an improved method of forming such a to protective gas by reacting a predetermined mix- 'heretofore been applicable generally to all types ci heat treatment which it may be desirable to carry out, as is the case with the furnace atmosphere produced according to the present invention. A

Hydrogen has been used in the heat treatment of steel and copper. One of the greatest objections to the use of hydrogen or mixtures containing a large percentage of hydrogen is the danger of explosion. Where it is used in the heat treatment of copper alloys, the hydrogen may combine with the oxides which are generally present in the copper alloys and render the metal brittle. Steam has been used in the treatment of copper alloys, but it is objectionable for the reason that it may produce stains. Nitrogen, although an inert gas, has not proved successful for the heat treatment of metals because the metals generally contain sufficient occluded oxygen to cause oxidation or staining of the metal even if the metal is enveloped in an atmosphere of nitrogen. Natural gas or other similar hydrocarbon gas, when heated to the temperatures usually employed in heat treating metal, decomposes and deposits carbon on the work and on the furnace interior, thus frequently short circuiting the resistors and producing other undesirable results such as the disintegration of the brickwork.

Mixtures of steam and gas have been reacted to form a furnace atmosphere containing about 75% hydrogen. The gas so produced, since it contains a very large percentage of hydrogen,l

is rather explosive, and this has seriously limited its use. Furthermore, the character of the furnace atmosphere produced by mixing steam and gas can be varied only over a relatively narrow range, so that its use is limited to the heat treatment of metals which require the presence of a furnace atmosphere containing around 75% hydrogen. Where it is attempted to reduce the hydrogen content in order to reduce the danger of explosion, carbon deposits, thereby decreasing the eficiency of the apparatus used in forming the furnace atmosphere and rendering it necessary at times to shut down the apparatus and clean it out.

It is well known that there are many problems involved in the successful use of the protective gasesl of the gaseous mixtures before mentioned. For instance, hydrogen or mixtures containing large amounts of hydrogen, such as the mixtures produced by dissociating ammonia, are very actively reducing When in contact With metallic oxides at elevated temperatures, and this is usually desirable because previous treatment generally results in more or less oxidation, which it is desired to remove as an annealing treatment. l-lowever, it is impossible to obtain satisfactory results in the annealing or other treatment of high carbon steel in an atmosphere of hydrogen because of the decarburizing effect of this gas. Hydrogen, therefore, either when used f pure or when mixed with an inert constituent such as nitrogen, is coniined to the treatment o'f metals Where the retention of a comparatively high carbon content is unnecessary. it will be readily seen that carbon steels cannot oe hardl ened from a hydrogen atmosphere in spit the fact that hardening Without surface oxidation is a very desirable process.

According to the present invention, hydrocarbon-containing gas, such, for example, as natural gas, is mixed with air in predetermined proportionsand the mixture is reacted in a reaction chamber heated from an independent source such as electrical resistors surrounding the reaction chamber. The reaction is carried out preferably in the presence of a catalyst. The temperature of the reaction chamber is preferably controlled automatically so as-to produce a furnace atmosphere of the desired character, and the reaction products are then cooled and dried and then used as an envelopingl atmosphere for the work being heated. The furnace atmosphere so produced can be varied in character according to the particular material which i s being heat treated and according to the Civ portions of air and gas supplied to the reaction chamber, the character of the furnace atmosphere issuing from the reaction chamber may be controlled. Where a relatively large proportion of 'air to gas is used, heat is supplied by the electrical resistors to start the reaction, and the resistors may then be cut out. Under certain conditions, it may be necessary to cool the reaction chamber, as by admitting a current of cold air around it, in order that the heat of the reaction may not produce an excessive temperature. With the lower ratios of air to gas, it is advisable to supply heat by the resistors throughout the entire reaction. In the reaction chamber, the carbon formed by dissociation of the hydrocarbon gas combines with the oxygen supplied by the air to form carbon monoxide and carbon dioxide, the proportions of these gases as well as the other gaseous constituents of the furnace atmosphere depending upon the proportions of air and hydrocarbon gas fed into the reaction chamber and also upon the temperature of the reaction chamber and the type of hydrocarbon gas which is mixed with the air.

Hydrogen is formed from the dissociation of the hydrocarbon, and a certain proportion of methane also remains as such, depending upon the temperature in the reaction. chamber. In addition to these constituents of the furnace atmosphere, there is a large proportion of nitrogen derived from the air. Usually the nitrogen will constitute about 35% to 80% of the furnace atmosphere. This large proportion of nitrogen greatly reduces inammability of the furnace atmosphere, thereby reducing the explosion hazards. It furthermore, furnishes a large body of an inert gas which acts as a stabilizer for the furnace atmosphere so that the furnace atmosphere may be accurately controlled by increasing or decreasing the percentages of active gases such as lrydrogen and methane. By suitably regulating the proportions of air to gas fed to the reaction chamber and by controlling the temperature of the reaction chamber at the proper value, the nature of the gaseous product may be varied over a wide range. Thus, an enveloping atmosphere suitable for many different treatments and processes may be produced. At all ratios wherein there is a substantial deficiency of air as compared with that necessary for perfect combustion of the gas, the reaction product is reducing or at least non-oxidizing. When the mixture fed to the reaction chamber contains a small proportion of air, the resulting gaseous product will be carburizing and deoxidizing in character. With a slight increase in the amount of air, the carburizing property is decreased or entirely lost while the reducing property remains. With further increase in the amount of air, the reducing property still remains, but the gaseous product becomes more or less decarburizing. Finally, when the amount of air is nearly or quite that required for perfect combustion, the gas becomes oxidizing in character. Thus it will be seen that a wide Variety of applications, particularly in the treatment of ferrous metals, is rendered possible by the control of the air-gas mixture fed to the reaction chamber.

An important advantage of my invention is that it permits an excellent protective gas to be manufactured at a very low cost. In most practical furnaces where protective atmospheres are used a certain minimum gas ow is neces- 1,979,820 particular heat treatment. By varying the prosary in order to prevent air from dill'using toward and injuring the work under treatment. It is for this reason that a cheap gas is-essential in order that the processes may be carried out economically. In my process, not only is the original hydrocarbon-containing gas much cheaper than any form of known protective gas such as hydrogen, dissociated ammonia or the like, but it is rendered still cheaper by admixture of large quantities of air.

As previously stated, the mixture of hydrocarbon-containing gas and air is converted in a reaction chamber to form the furnace atmosphere, and the reaction chamber is heated -by a source of heat which is independent of the heat liberated in the reaction. The conversion of the gas and air mixture, therefore, does not depend upon the temperature resulting from the reaction but may be carried out at any suitable temperature for producing an atmosphere of the desired character. Where the conversion of the gas and air mixture into an enveloping atmosphere for the work to be heat treated is dependent upon the temperature produced by the combustion of the mixture itself, the character of the furnace atmosphere so produced is seriously limited. In order to sustain combustion, it is necessary to use not less than a certain proportion of air to gas, and unless this minimum amount of air is used, the temperature necessary for the reaction cannot be maintained.

In the accompanying drawings,

Figure 1 is a diagrammatic illustration of a gas converter and associated apparatus for forming the atmosphere which is supplied to a heat treating furnace ;v

Figure 2 is a diagrammatic illustration of one type of furnace to which the atmosphere may be supplied, although other types of furnaces may be employed; and

Figure 3 is a graph illustrating the compositions of different gaseous products which may be produced in the gas converter by varying the ratio of air and natural gas which is fed to the gas converter.

Referring more particularly to the accompanying drawings, a hydrocarbon-containing gas, such as natural gas, is supplied to a gas converter 2 through a pipe 3 controlled by a valve 4. Air is supplied through a pipe 5 controlled by a valve 6, and after mixing with the gas, is delivered to a reaction chamber formed by a nickel-chromium alloy,tube 8 mounted within the gas generator, the mixture of gas and air within the tube 8 being heated by an electrical resistor 9. The temperature within the reaction chamber or tube 8 is maintained at about 1800-1900 F. by suitable control devices, and the reaction between the natural gas and air is aided by a catalyst such as expanded nickel sheet 10. The means for controlling the temperature may comprise a pyrometer tube' 1l inserted in the converter and connected by wires 12 to a recording pyrometer 13. The pyrometer is connected by wires 14 to a control device 15 which controls the current supplied by the conductors 16 to the lead wires 1'7 which are connected to the electrical resistor 9. The gas issuing from the reaction chamber is led through a pipe 18 to a cooler 19, and after removing condensed moisture in a moisture trap 20, ows through drying material in a drier 21. The drying material may be calciumV chloride, activated alumina, or other suitable material.. .The dried gas is delivered through 150 ihbda@ a pipe 22 to a pipe 23extending through the wall of a heat treating furnace indicated generally by the reference numeral 2i. The furnace is also provided with a restricted outlet opening 25, by means of which the character of the gas in the furnace may be'determined.

The atmosphere produced according to the present invention is applicable to heat treating furnaces in general, but for purposes of illustration, there is shown in Figure 2 a furnace having a heating chamber 26 and a cooling chamber 27 connected end to end, the opening therebetween being controlled by a damper 28. The heating chamber is heated by electrical resistors 29, and the work being heat treated is conveyed through the furnace on rollers 30. Adjacent each end of the furnace are vertical elevator shafts 31 and 32 in which elevators 33 and 34 are adapted to be raised and lowered by means of hydraulic cylinders 35 and 36. The work is conveyed to the vertical shaft 31 on rollers 37, the vertical shafts 31 and 32 being provided -with side doors (not shown) so that the material can be delivered to and from the shafts. The openings between the furnace and the vertical shafts are inthe bottom of the furnace. This tends to prevent air from entering the furnace since the protective atmosphere used in the furnace is lighter than air, and air progresses toward the interior of the furnace only by diffusion, which is relatively slow. The work is lifted by the elevator 33 to the level of the rollers 30 in the heat treating'furnace, and then a pusher mechanism 38 is operated to push the material or,tray on which it is supported onto the rollers 30. The rollers 30 are driven by means (not shown), and after the material has been heat treated, it is lowered on the elevator 3a and delivered onto rollers 39. During its passage through the furnace, the Work is enl veloped in the gaseous atmosphere produced in the reaction chamber of the gas converter.

Referring to the graph shown in Figure 3, the composition of the gas produced in the gas converter is shown for different proportions of air to natural gas fed into the gas converter. .The graph represents the results obtained by using natural gas containing about 84% methane, 14% ethane and 2% nitrogen, mixed with diiferent proportions of air up to a ratio of 10 parts of air by volume to 1 of gas. The .temperature in the reaction chamber was maintained at about i850" F. and expanded nickel sheet was used as the catalyst. The gases produced in the reaction chamber were analyzed to determine their composition, the percentages of hydrogen, methane, carbon monoxide, carbon dioxide, and nitrogen being given on the chart. It `will be seen that as the ratio of air to gas increases, the hydrogen, carbon monoxide and methane decrease, while the carbon dioxide and nitrogen increase. The nitrogen is inert so far as its action upon the steel or other material being heat treated is concerned. The hydrogen reduces oxides, forming Water vapor, and also tends ,to decarburize iron or steel. It is believed that some methane is formed by the reaction of the hydrogen and the carbon in the steel. The carbon monoxide also reduces oxides, forming carbon dioxide, while carbon dioxide has a tendency to produce oxides. The tendency toward decarburization depends, it is believed, to a great extent upon the balance between hydrogen on the one hand and carbon monoxide and methane on the other. This belief is supported by the fact that have demonstrated that with rich mixtures, where comparatively large amounts of methane and carbon monoxide are present, the gas is carburlzing in character, while with large amounts of air the methane practically disappears and the carbon monoxide is much smaller in amount, with the result that such atmospheres are actively decarburizing. For a given percentage of carbon in the steel a certain tendency to carburize will be necessary in order to prevent decarburlzation.

Thus, when it is desired to anneal pieces of different carbon content without change in their carbon content, it is necessary to control the character of the enveloping gas to produce a condition of equilibrium.

It has been-found that by mixing the air and I lgas in predetermined proportions, the effect of the furnace atmosphere upon the work can be controlled. Carbon monoxide and carbon dioxide have opposite effects upon the steel, the rst l tending to reduce oxides and the second tending to oxidize. In addition, carbon monoxide tends to carburize. Similarly, hydrogen and methane have opposite effects as relates to carbon. in the steel, the hydrogen tending to decarburize and the methane to carburize. By varying the ratio of air to gas, the constituents of the gaseous product produced in the gas converter can be varied so as to give the type of gaseous product best adapted for the particular use. Since the gaseous product is produced in an independent gas converter from which outside air is excluded, the mixture of air and gas in the converter can be accurately controlled and, therefore, the composition of the gas issuing from the converter can be accurately controlled. The gas produced in the converter is supplied to a heat treating furnace which is sealed from the outside atmosphere by some means other than the gas itself. Therefore, after the air which was present at rst in the heat treating furnace has been expelled, the atmosphere within the heat treating furnace can be maintained of the desired oomposition.

The proportions of air to gas supplied to the gas converter ,will depend, among other factors, upon the analysis of the gas which is mixed with air, the reaction temperature in the converter, the character of the material being heat treated and whether it is desired to have a carburizing, decarburizing, reducing or oxidizing atmosphere. if pure butane is used in place of natural gas of the composition given, the ratio of air to gas will be approximately three times as great as that given in the chart for any given composition of gaseous product which it is desired to secure. If three parts by volume of air are mixed with one part by volume of natural gas containing about 844% methane, 14% ethane and 2% nitrogen, the gas issuing from the converter will contain about li'1% nitrogen, 31% hydrogen, 19% carbon monoxide, 2% methane and 1% carbon dioxide. Ef butane is used in place of natural gas and it is desired to produce a gas of approximately the same composition, it will be necessary to use an Thus, if a low carbon steel containing .15% to mit .20% carbon is being annealed, it is advisable to use an atmosphere containing about 25% hydrogen, about 1% methane, about 15% carbon monoxide, and about 3% carbon dioxide, the remainder being nitrogen or other inert employed.-

gas. Gas of this composition can be obtained yby using air and natural gas of the composition given in theratio of about 4:1. It it is desired to anneal a steel containing about .50% carbon, a lower ratio of air to natural gas is employed, for example, about 2 parts of air to l of gas, as this increases the proportion of methane which remains undecomposed, thereby reducing the tendency of the steel to decarburize. Generally speaking, the higher the carbon content of the steel is, the higher the percentages of methane and carbon monoxide should be, and this means that as the percentage of carbon in the steel is increased, the ratio of air to gas should decrease in cases where it is desired to anneal without substantial decarburization.

The atmosphere produced in the gas converter may be either carburizing, neutral or decarburizing, according to the proportion of air and gas used. If it is desired to carburize a steel containing, say, .15% to .20% carbon, where a natural gas of the composition already given is employed, a ratio of air to gas lower than 4:1 is employed, for example, a ratio of about 3:1. This produces a gaseous product having higher content than is produced bya mixture of 4 parts of air and 1 part of gas, which mixture, as above stated, is suitable for Vannealing a low carbon steel where it is not desired to change the carbon content .of the steel. In the present instance, however, it is desired to increase the carbon content, and forthis reason a lower ratio' of air tovgas than 4:1 should be employed.

On the other-handyif it is desired to decarburize the steel containing .15% to .20% carbon,a higher ratio lof air to gas than 4:1 is If' the steel contains about .50% carbon andV it is desired to anneal it without substantial carburization or decarburization, an air to .gas ratio of about 2:1 is used. If it is desired to decarburize this steel, a higher ratio of air'to gas, for example, 3 or 4 to 1, should be used. If it is desired to increase the carbon content of a steel containing about .50% carbon, a ratio of air to gas less than 2:1, for example, a ratio of air to gas of lttl, is employed. For annealing copper, an air to gas ratio of 8:1 has been found very satisfactory. The proportion of air may be decreased, however, in some cases to a ratio of 4 parts of air to l of gas. For annealingy brass, an air to gas ratio of 4:1 is preferred. It will, thus be seen that by suitably proportioning the quantities of air and gas which arereacted in the gas converter, atmospheres can be produced which range from reducing and carburizing to oxidizing and decarburizing in character.

Although the composition of the enveloping gas will vary according to the particular use, I have found that for many purposes, the enveloping gas should contain from 7% to 37% hydrogen, up to 10%- methane, vfrom 5% to 22% carbon monoxide, `up to 9% carbon dioxide, and nitrogen from' 35% to 80%. Preferably, the hydrogen is from 15% to 35%, the methane from .5% to 5%, the carbon monoxide from 10% to 20%, up to 6% carbon dioxide, and the nitrogen from 40% to 68%. It will be seen that there is always a large proportion of nitrogen.

methane and carbon monoxide,

'I'he ,nitrogen reduces the danger of' explosion, and because of its large amount and inert character, the character of the whole gaseous mixture can be accurately controlled by varying the percentages of the more active constituents such as hydrogen and methane. Furthermore, the nitrogen acts as an inert diluent, which, having been obtained from air, costs nothing except the small expense of pumping.

The graph shown in Figure 3 represents in full lines the compositions of the gases produced in the reaction chamber when the nickel catalyst was used. The graph also shows fragmentary curves in dotted lines indicating the compositions of the gases when no catalyst was used in carrying out the reaction.

, The invention is applicable to heating processes in general and is not limited to the type of furnace shown. Hydrocarbons otherk than natural gas may be employed, and the ratios of airto hydrocarbon may be varied to suit the particular conditions.

It will also be understood that certain processes involving the heating of metals Without the use of afurnace may also be carried on advantageously-with the protective gas produced according to myinvention. For instance, this gas may be used in arc weldingresistance welding and the like, in which no fur'nace is necessary.

Although the invention has been described particularly in connection with the heat treatment of ferrous metals, it can be utilized with non-ferrous metals, and the invention may be otherwise embodied or practiced Within the scope of the following claims.

I claim:

1. The method of heat treating, comprising forming a mixture consisting essentially of hydrocarbon-containing gas and air inl predetermined proportions, reacting the mixture in a reaction chamber heated by external means to produce a gaseous product containing a predominant proportion of nitrogen and smaller amounts of hydrogen, methane, and carbon monoxide, and using the gaseous product in a heat treating chamber as an enveloping atmosphere for the material being heat treated.

2. The method of heat treating, comprising forming a mixture consisting essentially of hydrocarbon-containing gas and air in predetermined proportions, reacting the mixture in a reaction chamber heated by external means to produce a gaseous product containing from 35% to nitrogen and smaller amounts of hydrogen, methane, and carbon monoxide, and using the gaseous product in a heat treating chamber as an enveloping atmosphere for the material being heat treated.

3. The method of heat treating, comprising forming a mixture consisting essentially of hydrocarbon-containing gas and air in predetermined proportions, reacting the mixture in a reaction chamber heated by external means to produce a gaseous product containing from 7% to 37% hydrogen, up to 10% methane, from 35% to 80% nitrogen, and from 5% to 22% carbon monoxide, and using the gaseous product in a heat treating chamber as an enveloping atmosphere for the material being heat treated.

WILBUR S. BOWLING. 

